docs: def set_dtreat_indch(self, indch=None):
          """ Store the desired index array for the channels
  
          If None => all channels
          Must be a 1d array
  
          """
          if indch is not None:
              indch = np.asarray(indch)
              assert indch.ndim==1
          indch = _format_ind(indch, n=self._ddataRef['nch'])
          self._dtreat['indch'] = indch
          self._ddata['uptodate'] = False